This invention relates to protecting surfaces using a pressure sensitive adhesive article such as a masking tape, particularly at high temperatures.
Masking tapes are used in a variety of applications involving exposure to high temperatures on the order of 150xc2x0 C. or higher. For example, masking tapes known as lead frame tapes are used in the electronics industry to protect copper alloy lead frames against epoxy potting compound flash. As another example, masking tapes known as flash breaking tapes are used in the aerospace industry both for metal-to-metal bonding and composite bonding to help control the spread of bonding adhesives.
To be successful, these tapes must exhibit good adhesion to the desired substrate at high temperatures, yet be cleanly removable at the conclusion of the high temperature exposure to avoid leaving residue or other contaminants on the substrate surface. For example, in applications involving micro chip scale processing, the adhesion must be sufficient to prevent liquid epoxy resin from flowing into the interfacial region between the adhesive and the substrate to which the tape is adhered. Contamination is particularly a problem in applications involving the electronics industry because the contaminant can interfere with subsequent processing steps.
Currently, tapes featuring silicone-based pressure sensitive adhesives are used for high temperature applications, due to their outstanding thermal stability. However, these tapes often either contaminate the surface or leave silicone residue following removal, requiring a separate cleaning step prior to further processing. It is very desirable to develop silicone-free pressure sensitive adhesives that can be exposed to 150xc2x0 C. to 230xc2x0 C. for at least 30 minutes without leaving any adhesive residue upon removing. It is also very desirable to avoid the cleaning step prior to further processing if the adhesives do not contaminate the surface following removal.
The challenges are that all acrylic polymer based pressure sensitive adhesives have relatively poor thermal stability and they start to degrade slowly upon exposing to high temperature such as above 150xc2x0 C., which reduces the cohesive strength of the adhesives and thus leads to adhesive residue problem.
The present invention is directed to acrylic pressure sensitive adhesives that can retain enough cohesive strength following high temperature exposure and are removed cleanly, i.e. with little or no contamination to substrate surface.
In general, the invention features a pressure sensitive adhesive composition that includes the reaction product of (A) a copolymer comprising the reaction product of (a) a (meth)acrylate ester of a non-tertiary alcohol in which the alkyl group contains between 1 and 14 carbon atoms, inclusive, and whose homopolymer has a glass transition temperature no greater than about 0xc2x0 C.; and (b) a carboxylic acid-functional, ethylenically unsaturated co-monomer; and (B) a bis-amide crosslinking agent having the formula: 
where R1 and R3 independently, are selected from the group consisting of H and CnH2n+1 where n is an integer ranging from 1 to 5, and R2 is a divalent radical selected from the group consisting of benzeno (xe2x80x94C6H4xe2x80x94), substituted benzeno, triazine, CmH2m where m is an integer ranging from 1 to 10, and combinations thereof. As used herein, a xe2x80x9ccopolymerxe2x80x9d refers to a polymer prepared by reacting two or more monomers.
The relative amounts of the co-monomer and the crosslinking agent are selected such that the ratio of the number of equivalents of amide groups to the number of equivalents of carboxylic acid groups is at least about 0.1. To determine this ratio, the weight of each component (co-monomer and crosslinking agent) in the composition is divided by its theoretical equivalent weight to obtain the number of equivalents of each component. The ratio of equivalents of amide groups to equivalents of acid groups is then calculated equivalents (amide groups) divided by equivalents (acid groups).
The adhesive composition comprises no greater than 10% by weight of a tackifier and no greater than 2% by weight of a plasticizer.
In one aspect, the relative amounts of these materials are further selected such that the composition, when applied to a glass substrate at a thickness of 0.0008 inch and exposed to a temperature of 180xc2x0 C. for 30 minutes, is cleanly removable following heat exposure. In a second aspect, the relative amounts of these ingredients are further selected such that the composition, when applied to a copper-containing substrate at a thickness of 0.0008 inch and exposed to a temperature of 180xc2x0 C. for 30 minutes, is cleanly removable following heat exposure. By xe2x80x9ccleanly removablexe2x80x9d it is meant that following removal from the substrate, the substrate is substantially free of adhesive residue. Removability may be assessed according to the Test Methods, described below. In general, the lack of adhesive residue is determined visually with the unassisted eye.
The invention further features adhesive articles, such as tapes, based upon this adhesive composition, as well as a method for protecting the surface of a substrate using the adhesive article. According to this method, the adhesive article is applied to a substrate, after which the article and substrate are heated at a temperature ranging from about 150xc2x0 C. to about 230xc2x0 C. These conditions are representative of the conditions employed in processes used in the electronics industry. At the conclusion of the heating process, the article is cleanly removed from the substrate.
The invention provides pressure sensitive adhesive compositions, and articles that include these adhesives, that are thermally stable and have sufficiently high cohesive strength, even after high temperature exposure, thereby allowing them to be cleanly removed from a variety of substrates. These compositions are also chemically inert towards epoxy molding compounds. This feature makes them particularly useful as lead frame tapes because they do not adhere permanently to the epoxy molding compound that flows between the adhesive and the substrate on which the adhesive is placed during processing.
The details of one or more embodiments of the invention are set forth in the description below. Other features, objects, and advantages of the invention will be apparent from the description and from the claims.